A driver driving a vehicle such as an automobile has to safely and quickly perform various driving operations such as understanding what happens outside the vehicle, reading information from display devices of the vehicle and performing driving actions. Therefore, it is desirable that vehicle information sent from the display device is readable in a necessary movement range of the driver's viewpoint for understanding circumstances outside the vehicle while driving. For example, an image display device desirably irradiates light onto a part of a transparent plate such as the front windscreen of the vehicle to display texts and/or images.
Various devices such as a head-up display (hereinafter, called “HUD”), which displays driving information on a front windscreen of an automobile or a head-mounted display (hereinafter, called “HMD”), which displays information on a lens section of an eyeglass apparatus, are exemplified as such a transmissive display device. A driver uses such a transmissive display device to see driving information (e.g., a map and/or a speedometer) with an ambient outlook. Therefore, it is expected that such transmissive display devices contribute to safe driving operation of the driver.
FIG. 46 shows a schematic view of a conventional HUD. According to Patent Document 1, the HUD projects a virtual image onto the front windscreen. A highly transmissive hologram combiner is attached to the front windscreen. The hologram combiner transmits external light at a high transmission ratio and also directs very bright display image, which is projected onto the hologram combiner, to the driver's eyes. The external light passes through the hologram combiner at a high transmission ratio and reaches the driver's eyes.
As shown in FIG. 46, the HUD mounted on the vehicle 101 comprises an HUD optical unit 102, which is stored in the dashboard, a display part 103 and a deflecting element 104 which are situated inside the HUD optical unit 102, a light source (not shown), which is situated inside the display part 103, and a combiner 106, which is attached to the front windscreen 107.
For example, the display part 103 includes liquid crystals to display information (an image), which a driver DR watches. The display light, which represents the image generated by the display part 103, is projected towards the deflecting element 104. For example, the deflecting element 104 includes optical elements such as mirrors to deflect the display light from the display part 103 toward the combiner 106, which is attached to the front windscreen 107.
An opening 105 is defined on the HUD optical unit 102. The deflected display light from the deflecting element 104 is projected through the opening 105 onto the combiner 106 which is formed on a surface of the front windscreen 107. The front windscreen 106 reflects the display light from the HUD optical unit 102 toward the driver DR.
The driver DR may see the reflected display light from the combiner 106 to visually obtain driving information. The eye box EB means a range in which the driver DR may see an image when the driver DR moves the head. If the eyes of the driver DR stay inside the eye box EB, the display light reaches the retina of the driver DR, so that the driver DR may view the image. The combiner 106 has characteristics, which allow transmission of the external light outside the front windscreen 106 (the exterior) and reflect the display light from the HUD optical unit 102. Due to these characteristics of the combiner 106, the driver DR may see an ambient view outside the front windscreen 107 together with the displayed information from the display part 103. The driver DR may use an HUD to see information from the display part 103 without moving the sight line away from the exterior. Since the driver DR moves the sight line less frequently, the driving operation becomes safer.
According to Patent Document 1, a hologram combiner is used as the combiner 106. A hologram combiner has high wavelength selectivity. For example, the hologram combiner reflects only wavelengths used by the display part 103 and transmits other light wavelengths. Therefore, the hologram combiner may reflect bright light from the display part 103 and simultaneously transmit external light at a high transmission ratio.
According to Patent Document 1, a concave mirror is used as the deflecting element 104. Patent Document 1 proposes usage of a concave mirror to correct resultant image distortion from a curvature of the combiner 106.
Adjustment to an incident light angle on a hologram used as the combiner 106 and an angle of diffracted and emitted light from the combiner may be attempted in order to make resultant aberration from the hologram less influential (c.f., Patent Document 4).
The HUD shown in FIG. 46 uses the combiner 106 (hologram combiner) attached to the front windscreen 107 to present an image to the driver DR. However, the display light from the display part 103 is also reflected not only by the combiner 106 but also by the front windscreen 107. Therefore, the driver DR sees images created by the diffracted display light from the combiner 106 and by the reflected display light from the front windscreen 107.
FIG. 47 is a schematic view, which describes a problem caused by the reflected display light from the front windscreen. The problem caused by the reflected display light from the front windscreen is described with reference to FIG. 47.
FIG. 47 shows two glass plates (an inner glass plate 202 and an outer glass plate 203), which are used as a front windscreen, and a hologram combiner 201, which intervenes between the inner and outer glass plates 202, 203. Display light (incident light IL) is projected towards the hologram combiner 201. The hologram combiner 201 diffracts the incident light IL to create diffracted light DL towards the driver DR.
The inner glass plate 202 forms an inner surface of the vehicle whereas the outer glass plate 203 forms a boundary surface between the vehicle and the exterior. The inner glass plate 202 reflects the incident light IL, which is directed to the hologram combiner 201, towards the driver DR. The reflected light from the inner glass plate 202 is indicated as the reflected light RL1 in FIG. 47. The outer glass plate 203 reflects the light, which passes through the hologram combiner 201, towards the driver DR. The reflected light from the outer glass plate 203 is indicated as the reflected light RL2 in FIG. 47.
The driver DR views an image displayed by the diffracted light DL from the hologram combiner 201, an image displayed by the reflected light RL1 from the inner glass plate 202, and an image displayed by the reflected light RL2 from the outer glass plate 203. Consequently, the image displayed by the display light from the display part is viewed by the driver DR as blurred double or triple images.
Hereinafter, in the present specification, a problem which causes drivers to simultaneously view the diffracted light from the hologram combiner and the reflected light from the glass plates (front windscreen) is called “surface reflection problem”.
FIG. 48 shows a schematic view of a method for solving the surface reflection problem. The method shown in FIG. 48 resolves the surface reflection problem by making a directional difference between the diffracted light from the hologram combiner and the reflected light from the surface of the glass plates.
Optical characteristics of the hologram combiner 201 shown in FIG. 48 are designed so that the diffracted light DL is emitted at a diffraction angle DA in response to the light (incident light IL) which is incident at an incident angle IA. In this case, the reflected light RL1 from the inner glass plate 202 and the reflected light RL2 from the outer glass plate 203 are emitted from the front windscreen at an emission angle OA which is equivalent to the incident angle IA. If there is a sufficiently large difference between the diffraction angle DA and the emission angle OA, only the diffracted light DL from the hologram combiner 201 is incident on the eye box EB in which the driver DR sees the display image. Consequently, the driver DR is less likely to view the reflected lights RL1, RL2 from the inner and outer glass plates 202, 203.
For example, if the emission angle OA of the reflected lights RL1, RL2 is sufficiently larger than the diffraction angle DA as shown in FIG. 48, the diffracted light DL is incident on the eye box EB whereas the reflected lights RL1, RL2 deviate over from the eye box EB. For instance, Patent Document 2 proposes a display device for a vehicle which causes a directional difference between the reflected light from a glass plate and the diffracted light from a hologram combiner according to the scheme shown in FIG. 48.
Patent Document 3 proposes a hologram applied to other portions than a combiner for quality improvement of images presented to a driver by an HUD. If a hologram is used as the deflecting element 104 to deflect the light from the display part 103 of the HUD described with reference to FIG. 46 towards the combiner 106 instead of a mirror, the resultant aberration from the combiner 106 becomes less influential.
A large difference between the diffraction angle and the emission angle resolves the surface reflection problem but causes image blurring (color aberration) because of a wavelength width of the light source. Here, “image blurring” means phenomena in which incident light is diffracted in a different direction from a designed direction for a designed light wavelength of the hologram combiner if different light in wavelength from the design wavelength is incident on the hologram combiner.
If the hologram combiner is designed on the basis of the central wavelength of the light source and if the wavelength width of the light source becomes wider, a larger amount of light which has different wavelengths from the designed wavelength is incident on the hologram combiner. Consequently, a large amount of the light from the light source is diffracted in a different direction from the designed diffraction angle. The greater the difference between the incident angle and the diffraction angle of the hologram combiner, the more influential the image blurring caused by the hologram combiner. Consequently, if the wavelength width of the light source is large, a large difference between the incident angle and the diffraction angle of the hologram combiner largely reduces image resolution, which is presented to the driver, although the difference resolves the surface reflection problem. Patent Document 2 does not take account of the wavelength width of the light source.
A hologram used as the deflecting element 104 makes the resultant image blurring from the combiner 106 less influential. However, the optimum optical design for making the image blurring less influential (e.g., settings of the optimum incident angle and diffraction angle for the deflecting element 104 which uses a hologram) depends on the wavelength width of the light source and the target resolution. Patent Document 3 does not take account of this point.
Usage of an optical element having magnification characteristics such as a hologram or a concave mirror as a combiner allows a compact design of an HUD. However, if an optical magnification ratio of the combiner is increased, the optical system causes aberration large enough to decrease resolution of the displayed image.
Patent Document 4 proposes setting the incident angle and the diffraction angle (emission angle) at the combiner to 20° or more to resolve the aforementioned problem. However, if there is a large difference between the incident angle and the emission angle at the combiner and if the light source supplying light to the display element to display an image has a large wavelength width, the hologram causes the image blurring. Patent Document 4 does not take account of the wavelength width of the light source.
Patent Document 1 proposes a turning mirror, which also works as a concave mirror, to correct resultant distortion from a combiner. According to Patent Document 1, a decrease in resolution is prevented by the concave mirror function. However, if a concave mirror is simply used, astigmatisms caused by the combiner and aberration because of the concave mirror are superimposed on each other. Therefore, resultant image resolution may not be improved. A reduction in a magnification ratio of the optical system may improve the image resolution but the optical system undesirably becomes larger for application to transmissive display devices such as HUDs which are mounted in small vehicles.
Patent Document 1: JP 3,418,985 B
Patent Document 2: JP 2,751,436 B
Patent Document 3: JP H06-167671 A
Patent Document 4: JP H10-010464 A